The present embodiment relates to color consistency and gray balance calibration in an integrated printing system. It finds particular application in conjunction with a system for increasing the consistency between the outputs of multiple marking engines, and will be described with particular reference thereto. However, it is to be appreciated that the present embodiment is also amenable to other like applications.
In general, computers and other electronic equipment generating and inputting color images or documents typically generate three-dimensional or RGB (red, green, blue) color signals. Electrographic devices, such as printers, copiers, and the like, however, often print in four-dimensional or CMYK (cyan, magenta, yellow, and black) colors (and often can also receive such signals as input). A look-up table is commonly provided to convert each digital RGB color signal value to a corresponding digital CMYK value before or after being received by the printer. Due to the nature of printing inks, and their light absorption characteristics, however, complex non-linear colorimetric relationships exist between the input and output values. Typically, a color correction look-up table is built which approximates the mapping between RGB colorimetric space and CMYK values. The color correction look-up table may be generated by sending a set of CMYK digital values to the printer, measuring the colorimetric RGB values of the resulting color patches outputted by the printer, and generating the look-up table from the difference between the inputted values and the measured outputted values. More specifically, the color correction look-up table corrects for non-linearities, printing parameter variations, and unwanted absorptions of inks, so that the printer will print the true corresponding color. The color of the patches is typically measured with a high accuracy spectrophotometer, or the like. Such systems are described, for example, in U.S. Pat. No. 6,157,469 to Mestha; U.S. Pat. No. 6,384,918 to Hubble, III, et al.; U.S. Pat. No. 6,584,435 to Mestha et al.; U.S. Pat. No. 6,721,692 to Mestha et al.; above-referenced application Ser. Nos. 10/673,688 and 09/487,586, the disclosures of which are incorporated herein in their entireties by reference.
Over time, the output of conventional printers drifts (or deviates from predetermined optimum standards) due to various factors. These factors include environmental conditions (temperature, relative humidity, etc.), use patterns, the type of media (e.g., different paper types, transparencies, etc.) used, variations in media, variations from original models used in initialization, general wear, etc. To correct for the drift, the system is adjusted or recalibrated periodically. Recalibrating the color correction table involves periodically printing and remeasuring a set of test color patches which are then compared to an original set of color patches by calibration software.
Spectrophotometers have been used in a feedback loop of an on-line color correction system. Such a spectrophotometer may be periodically recalibrated, to ensure its accuracy, as described, for example, in U.S. Pat. Nos. 6,157,469 and 6,351,308 to Mestha, the disclosures of which are incorporated herein in their entireties, by reference. Calibration systems of this type use a reduced (smaller) number of color patch samples, printed at intervals during the regular printing operation of the printer, yet still provide relatively substantially continuous updating correction of the printer's color renditions over a wide or substantially complete color spectra.
U.S. application Ser. No. 09/566,291, noted above, discloses an apparatus for automatically calibrating a digital printing system. The apparatus utilizes a limited dynamic color balance control system using an in-line spectrophotometer color measurement system in the output path of a color printer for measuring colors on printed test sheets/banner pages without requiring manual operation or operator input. The automatic color balance control system produces tone values for all four primary colors by printing patches, measuring colors and automatically readjusting the tone reproduction curves until a satisfactory level of accuracy is obtained. While producing color balanced Tone Reproduction Curves (TRCs), the system will automatically lock the printer output to some predetermined color patch targets. In one embodiment, this output is locked to neutral gray when target colors are set to neutral gray inside the digital front end (DFE). After converging to the targets, the control system will return full TRCs for use inside the normal print path. The process is enabled either by the system controller or by the user with minimal intrusion. The control system uses control algorithms to achieve greater accuracy in the presence of uncertainties in the printing system.
Systems which employ several small printers are now being developed. These systems enable high overall outputs to be achieved by printing portions of the same document on multiple printers. Such systems are commonly referred to as “tandem engine” printers, “parallel” printers, or “cluster printing” (in which an electronic print job may be split up for distributed higher productivity printing by different printers, such as separate printing of the color and monochrome pages and are then recombined to form a document.
The eye is sensitive to color and gray balance variations in printer outputs, particularly when the outputs form facing pages in a document. Thus even small differences between outputs from different printers which are destined to be closely positioned within a document can be considered a defect. To add to the complexity of color matching between color image forming devices, different color image forming devices can use different types of toners, dyes, pigments, or inks to produce the outputted color images. Likewise, the color images can be produced on a wide range of copy media. Images can be produced, for example, on copy media ranging from paper to plastic, from fabric to metal. In each case, each combination of colorant and media produces a different optical appearance.
In an attempt to solve the problem of color matching of pages produced by different printers, various color matching techniques have been developed that use models to translate colors from one color space to another color space. These models usually manifest themselves in the form of predetermined multi-dimensional look-up tables.